Using rapid flow microfluorometry for analysis and sorting of cells, aspects of the following major subject areas have been supported during the previous year: (1) expression of cell surface differentiation markers on subsets of human T lymphocytes; (2) analysis of murine T cell differentiation markers during fetal development, during thymus repopulation following irradiation, and following enzymatic stripping from cell surfaces; (3) analysis of IgM allotypes and their genetic control; and (4) analysis of antibodies induced by anti-idiotypes.